FETTKE, MANHATTAN SCHIST OF NEW YORK 211 



It was absolutely colorless. The serpentine to which it lias altered is also 

 colorless in thin section and grass green in the hand specimen. No other 

 minerals occur which would indicate the presence of much iron in the 

 original sediment from which the ophicalcite was derived, as for example, 

 phlogopite or biotite. Therefore, it appears probable that the original 

 mineral was monticellite. 



The true Manhattan schist overlying the limestone at this point is a 

 feldspathic micaceous variety of medium gray color. Under the micro- 

 scope, it is seen to consist largely of biotite, muscovite, quartz, feldspar, 

 mostly orthoclase, sillimanite and a little garnet. A few small rounded 

 grains of zircon are also present. The rock has undergone considerable 

 crushing here since the original recrystallization during metamorphism 

 took place. This is shown by the nature of the broken quartz and feldspar 

 crystals and to a less extent the mica. The mica also often occurs in bent 

 crystals. 



Another variation in the schist observed, especially in the southern 

 portion of the area orL Manhattan Island, and not heretofore described, 

 appears in the form of occasional bands very rich in cyanite. These sel^ 

 dom reach a width of more than an inch or two, and wherever observed 

 were parallel to the schistosity. At times, these bands are made up en- 

 tirely of long prismatic crystals of cyanite associated vsdth muscovite and 

 quartz ( PI. XII, Fig. 3 ) . These crystals are optically negative and show 

 elongation parallel to the slow ray. Extinction is unsymmetrical. Their 

 long axes are parallel to the schistosity. These bands grade into the mica 

 schist in which the cyanite occurs associated with biotite, muscovite, some 

 garnet and only a little quartz. Thin veinlets of introduced quartz are' 

 usually associated with the bands running parallel to the foliation. A 

 chemical analysis of this schist is given in a later paragraph under analy- 

 sis 5 on page 212. 



STRUCTURAL FEATURES ■ 



The schist and underlying formations, as has already been mentioned 

 in the introduction, occur in a series of rather closely folded anticlines 

 and synclines usually unsymmetrical and often overturned toward the 

 west. The axes of these folds run in a general northeast and sputhwest 

 direction and in many cases have a gentle dip toward the south. In addi- 

 tion to these major folds, many minor folds are developed in the schigt, so 

 that at times it becomes exceedingly contorted and crinkled. As is usually 

 the case in folded rocks of this nature, the axes of these minor folds are 

 parallel to those of the major ones. ^ 



Most of the schist, especially the more micaceous varieties, shows a 

 marked foliated structure. In the case of the more gneissoid varieties, 



